Nijmegen breakage syndrome (NBS) is characterized by extreme sensitivity of ionizing radiation and increased risk for cancer. Multiparameter flow cytometry, including BrdU content analysis, is being used to investigate the DNA damage responses, including cell cycle arrest, in primary NBS fibroblast cell line. Initial studies indicate that NBS cells show normal G1 and G2 arrest in response to ionizing radiation. In on going studies we will further examine the effect of radiation on other NBS cell lines with potentially different breakage syndromes.